feat: Add --config flag for custom settings file path (#166)

* Add --config flag for custom settings file path

Allow users to specify an alternative settings file via `--config <path>`,
enabling distinct status line configs for different Claude Code instances.

- Add initConfigPath/getConfigPath/isCustomConfigPath to config.ts
- Fix backup path to always append .bak instead of replacing .json suffix
- Use path.dirname(SETTINGS_PATH) instead of CONFIG_DIR for mkdir
- Parse --config arg in ccstatusline.ts main() entry point
- Add tests for config path management

* Integrate --config flag with Claude Code install commands

When a custom config path is active, the install-to-Claude-Code feature
now appends --config <path> to the status line command. Also updates
isInstalled detection to recognize commands with the --config suffix.

- Add isKnownCommand() to deduplicate command-matching logic
- Add shell-safe path quoting for --config values with spaces/special chars
- Add buildCommand() to append --config when custom path is active
- Refactor isInstalled() to use isKnownCommand()
- Use isKnownCommand() in App.tsx install flow
- Add tests for isKnownCommand() and buildCommand() with CLAUDE_CONFIG_DIR safety net

* Show custom config path in TUI header

When a custom --config path is active, display it below the title bar
so users can see which settings file is being edited.

* fix: use platform-appropriate quoting for --config path

* chore: bump version to 2.1.8 and update recent updates

---------

Co-authored-by: Matthew Breedlove <sirmalloc@gmail.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "ccstatusline",
-  "version": "2.1.7",
+  "version": "2.1.8",
   "description": "A customizable status line formatter for Claude Code CLI",
   "module": "src/ccstatusline.ts",
   "type": "module",